1. Field of the Invention
The present invention relates to a photodetector.
2. Description of the Related Art
A number of photodetectors used for detecting an electromagnetic wave are known, and for example photodetectors having sensitivity to light from the ultraviolet range to the infrared range are collectively referred to as optical sensors. Among them, an optical sensor having sensitivity in the visible light range with a wavelength of 400 nm to 700 nm is particularly referred to as a visible light sensor, and a large number of visible light sensors are used for devices which need illuminance adjustment, control of an on state and an off state, or the like depending on human living environment.
For example, in a display device, ambient brightness can be detected by an optical sensor to adjust the display luminance. By detecting the ambient brightness and obtaining appropriate display luminance, the visibility can be improved and unnecessary electric power of the display device can be reduced. Examples of display devices including an optical sensor for adjusting display luminance are mobile phones and computers with a display portion. Further, in a display device, the luminance of a backlight can be detected by an optical sensor to adjust the luminance of a display screen. An example of a display device including a backlight is a liquid crystal display device.
In a device including the optical sensor, a photoelectric conversion element such as a photodiode is used in a portion for detecting light (also referred to as a sensor portion), and the intensity of light can be detected based on the amount of current that flows into the photoelectric conversion element when light enters the photoelectric conversion element. Patent Document 1 discloses an electric charge accumulation optical sensor having the following structure. As an analog signal, a comparator detects a voltage that is changed by discharging electric charge accumulated in a capacitor with a current which flows from a photodiode depending on the amount of incident light, by a constant current circuit (also referred to as a constant current source); and a counter circuit and a latch circuit generate a digital signal by using the time it takes for change of the voltage detected by the comparator.